This invention relates to an injection molding machine and is more particularly concerned with an openable and closeable injection nozzle mounted at the discharge end of the plasticizing cylinder which forms part of a plasticizing unit. The nozzle is axially coupled to the plasticizing cylinder and has a discharge opening which may be closed by a valve needle controlled by a power mechanism, such as a hydraulic piston-and-cylinder unit. At least one of force-transmitting linkage means comprises joints at both ends and is connected to the valve needle.
In known injection molding nozzles of the above outlined type, the valve needle is arranged coaxially with the valve body and may be operated by a control cylinder with the intermediary of a two-arm lever. Such an arrangement which is disturbing the thermal symmetry of the nozzle is shown, for example, in French Pat. No. 1,439,001 (FIGS. 7 and 8), in German Utility Model No. 7,020,969 (FIGS. 5-7) and in German published patent. application No. 23 36 099. Such injection molding nozzles are also known from U.S. pat. application Ser. No. 07/168,771now U.S. Pat. No. 4,808,103, corresponding to West-German patent application No. 37 08 434, which patents are hereby incorporated herein by reference.
It is a disadvantage of the above-outlined prior art arrangement that because of the use of a two-arm lever, the nozzle channel cannot be arranged coaxially with the axis of injection. Rather, the plasticized synthetic material is injected into the discharge zone of the injection nozzle from the plasticizing chamber of the plasticizing cylinder through detour channels, arranged radially offset to the injection axis. Because of the multiple change in the direction of flow of the plasticized material and because of the cross-sectional configuration of the detour channels, a significant pressure drop is usually experienced. When the needle valve of the nozzle is directly actuated by the control cylinder (as shown in FIGS. 5-7 of German Utility Model No. 7,020,969), the needle valve is in direct engagement with the piston of the control cylinder which is situated coaxially to the flow channel. The coaxial arrangement of the control cylinder, however, requires that the plasticizing cylinder be laterally offset relative to the discharge nozzle. Consequently, a throughout linear flow channel extending from the plasticizing chamber of the plasticizing cylinder to the nozzle outlet cannot be constructed regardless of whether the valve needle is actuated by means of a lever or directly by a coaxial control cylinder: the construction of such a nozzle channel is impeded either by the force-transmitting lever or by the control cylinder. Thus, for this reason, in each instance, flow channels are required which have portions oriented transversely to the axis of injection.
Furthermore, closeable injection nozzles are known, whose spring-biased valve needles are arranged at an angle to the injection axis, such as shown in FIG. 3 of French Pat. No. 1,439,001 and in German Utility Model No. 1,796,444. In such closeable nozzles, the nozzle valve opens when the pressure of the synthetic material exceeds a predetermined limit. Consequently, in such nozzles the preconditions for a control by computer program are missing.